1. Field of the Invention
The present invention relates to an improved differential pressure gage. More specifically, the present invention is directed to a new, novel differential pressure gage employing multiple coaxial measuring cylinder bores with associated reciprocal inner and outer measuring pistons.
2. Description of the Background
Accuracy in pressure measurement instruments is important for many applications such as flow measurement, petroleum reservoir analysis, medical gas testers and transducers, hydrostatic depth transmitters, and oxygen system testers. However, the accuracy of a pressure measuring instrument is limited by the accuracy of the pressure standard used for calibration of the pressure measuring instrument. Pressure standards of primary accuracy are traceable to those of the U.S. National Institute of Standards and Technology (NIST), formerly called the National Bureau of Standards. Pressure standards are used for several purposes including calibrating other pressure measuring instruments, directly measuring unknown pressures, and supplying reference pressures for various purposes.
The accuracy of piston-cylinder pressure gages used as primary standards is directly dependent on the precision of the size of their measuring piston or pistons and the corresponding measuring cylinders. Millionths of inches in tolerances will typically determine the accuracy of high quality pressure gages. It is necessary for the piston and annular area of the cylinder to have an extremely fine surface quality and uniform geometry over their entire length. At this level of accuracy, the existence of a fingerprint on the piston is a detectable error. The annular clearance between the piston and cylinder is in the range of five millionths of an inch. Since this clearance is so small, the piston-cylinder requires no additional seal between the piston and cylinder.
Due to the extremely fine tolerances of these piston-cylinder mechanisms, it is possible to use gas molecules rather than oil for lubrication between the piston and the cylinder. The lack of oil surface tension and friction enables the gage to operate with excellent repeatability as well as precision. In general, "gas-lubricated" pressure gages provide greater accuracy than hydraulic gages at pressures below 90 psi (630 kPa) and operate at pressures down to 0.2 psi (1.4 kPa) whereas oil gages are not desirable at pressures below 15 psi (105 kPa).
For low pressure accuracy, in which a tare error becomes significant due to the weight of the measuring piston itself, a low range piston may be made of 440 stainless steel to minimize mass of the piston and thereby reduce tare error. Tare error can be more precisely offset by use of a separate slave gage. Mid and high range pistons which are used to make measurements in pressure ranges where tare error is not as significant, are often made of tungsten carbide for optimum long-term stability, minimum distortion, and low thermal coefficient of expansion at high pressure. The corresponding measuring cylinders are also often made of tungsten carbide.
Masses used to activate the measuring piston-cylinders may be machined from nonmagnetic stainless steel for long-term stability and usually require serial numbers for identification purposes. In some cases, masses are provided with a calibration certificate traceable to the U.S. National Institute of Standards and Technology (NIST) per MIL-STD-45662 with each mass value reported to plus or minus 10 ppm or one milligram, whichever is greater.
Piston pressure gages have a number of common operational characteristics. When a pressurized fluid is applied to both ends of a piston, opposing forces of equal magnitude are exerted along the axis of the piston. When pressure is applied to only one end of the piston, the force is unbalanced and the piston moves in the opposite direction. When another force of equal magnitude, such as from a mass-load accelerated by gravity, is applied to the opposite end of the piston, the force becomes balanced and a differential pressure is established between the two ends of the piston. In principle, all piston pressure gages generate stable differential pressures by balancing the force caused by pressurization against another force such as that exerted by a mass-load in the presence of gravity.
Several terms associated with pressure metrology are used in describing measurements made with piston pressure gages. For instance, when the pressure acting on the mass-load end of the piston (reference pressure) is ambient pressure, the pressure at the opposite end of the piston is referred to as gage pressure. When the reference pressure is reduced to near zero, the pressure at the opposite end of the piston is referred to as absolute pressure. When the pressure at the end of the piston opposite the mass-load is at ambient atmospheric pressure and the pressure acting on the mass-load end of the piston is adjusted to some sub-atmospheric pressure, this sub-atmospheric pressure is referred to as negative gage pressure relative to the ambient atmospheric reference pressure at the opposite end of the piston. Although the operation of all piston pressure gages is technically differential, the term differential pressure generally refers to a measurement condition whereby the reference pressure is adjusted to some level other than near zero or atmospheric pressure. For differential pressure operation, the reference pressure is referred to as the line or static pressure.
Designs for most precision piston pressure gages are such that when generating absolute, negative gage, or differential pressures, a special chamber is installed surrounding the mass-load end of the piston so that the reference pressure can be adjusted. This design is time consuming to operate and generally requires rather elaborate mechanisms for mass manipulation (loading and/or rotation) while the chamber is installed.
Gravity acts not only on the mass-load, but also on the measuring piston, which also has a mass, and generates a downward force (in the direction of the force of gravity). When pressure is applied to the bottom of the piston to oppose the downward force from the mass of the piston alone, the pressure is known as tare pressure and is the minimum pressure obtainable without some special configuration. Numerous elaborate designs have been developed to eliminate or minimize tare pressure, yet it remains one of the significant obstacles in piston pressure gage metrology.
For differential pressure measurement, one presently available differential pressure gage uses a combination of oil and gas lubrication with the measuring piston-cylinders. This device employs three coaxially linked piston-cylinders. The middle piston-cylinder has an effective area 11 times or 101 times greater than the outer two pistons which are theoretically equal in area. An external oil pressure supply makes the mobile assembly, which includes the three pistons and their linkages, float when the value of the oil pressure acting on the lowest piston is 10 times or 100 times greater than the value of the unknown gas pressure.
While this device does produce a differential pressure, it has a number of significant problems. One major problem in using the three different pistons is in the design of linkages. These linkages are rather complicated, having up to four springs each, as well as plates, sockets, and spring struts. The linkages have a complex job to do in making up for slight errors in concentricity or coaxiality of the cylinders while still maintaining a strong enough mechanical link that the pistons move precisely in concert with each other. The linkages have weight which adds to tare error. Other problems include difficulties in making three separate pistons and cylinders with the proper relative dimensions at the level of accuracy required to avoid problems of leakage and friction. Friction wear is a problem that reduces the accuracy of the standard over time and may be accentuated due to any lack of perfection in concentricity of the cylinders with respect to each other.
Another significant problem with this type of differential pressure standard is the lack of versatility. Due to the construction of the instrument, the different size pistons in the instrument are not useable to produce dual ranges when operating the instrument to produce absolute or gage pressures. The necessity of requiring a separate pressure standard to activate the differential pressure standard is also a significant disadvantage of this machine. In producing differential pressures, it is necessary to restrict differential operation of one presently available multi-mode three-piston gage so one of the differential pressures is limited to being equal to or less than atmospheric pressure.
Two piston gages using either linkages between the pistons or telescoping sets of pistons having no linkages but rather having one measuring piston moving within another have been used to multiply, divide, and/or produce a wider range of reference pressures. A major disadvantage of these instruments, prior to this invention, is that it had not been determined how a two piston gage can be used to accurately produce or measure differential pressures. Another significant disadvantage that the two piston gages have had in the past is that in producing absolute reference pressures which require a vacuum, it has been necessary to place the masses within the vacuum.
The pressure gages described above fail to provide a sturdy differential pressure gage which has wide utility for all purposes including producing high and low range absolute and gage pressures, absolute reference pressures without breaking vacuum, negative gage pressures, and differential pressures when both static pressures may be above atmospheric pressure as well as when one or both differential pressures are equal to or below atmospheric pressure. Those skilled in the art have long sought and will appreciate the novel features of the present invention which solves these problems.